Prior to the advent of low wet pickup dyeing systems for carpet yarns, variations in heat-setting of such yarns have not presented serious problems. Traditional carpet dye systems heretofor produced good level dyeing notwithstanding slight heat set variations. However, traditional parameters such as good wet out, excessive dye/fiber liquor ratios and dwell time for dye strike completion are expensive and are under pressure to be modified for economic reasons. Low wet pickup has become increasingly important due to the energy cost of removing excess water from the dyed carpet. The lower the wet out the less energy require to dry the carpet. Low dye liquor ratios have become necessary due to the increasing cost of dye and the expense of removing residuals from waste water before the waste is accepted by city waste water systems. Traditional prior dyeing systems require time and considerable energy, but they do provide excellent quality carpet dyeing. Machine time of expensive new dye systems such as those using the curved blade applicator system and other systems constantly pressure dye houses to raise their speeds for higher production.
As continuous dye systems are modified for low wet pickup and speeds are increased for more production, strike rate becomes a very critical factor in the level dyeing of carpet. In the past, inherent designs of heat set machines have created strike rate variations which nevertheless produced satisfactory results because these were accompanied by excessive dye liquor ratios and time. These strike rate differences have gone unnoticed until the advent of the new dye systems. Strike rate variations result in variable dye uptake at high speeds which produces light dyeing intermittent areas within the end of yarn.
The present invention concerns an improvement to the construction and operation of continuous autoclave heat setting machines which include a heat recirculation system. Such machines have been in long continuous use to heat set yarn for carpets. The design includes a sealed tubular heat-set chamber approximately 16 feet long through which is driven a conveyer belt upon whose upper surface rests the yarn which is to be heat set. Steam is generated in the heat-set chamber and totally encompasses the conveyer belt and yarn thereon, the temperature of the chamber being maintained to the required heat set temperature which may be for example 270 degrees Fahrenheit. A forced continuous recirculation system takes the heating medium from a point adjacent to the exit of the heat-set chamber and returns such medium to a point adjacent to the entrance of such chamber from which it is directed through a plenum beneath the conveyor belt and therefrom, through the yarn.
However, upon the increase of yarn speeds through the heat-set chamber and the use of continuous dye systems for low wet pick up, it has been found that for reasons not understood by the equipment manufacturer or by the thousands of users of such equipment, dye streaking can occur which has become evident only after the carpet product has been produced. This has resulted in excessive seconds and the loss of a great deal of money for the carpet industry. It was suspected that this problem was related to the stopping of the running of the yarn through the heat-set chamber due to doffing or ends down and furthermore, that the problem might be associated with a temperature gradient in the heat-set chamber which normally did not affect yarn dye strike rate when the yarn is continuously run through the chamber. Upon investigation and testing, particularly of possible temperature gradients which might exist throughout the length of the heat-set chamber, it was discovered that these did in fact occur and exist. The present invention concerns an improvement which has been made to the design of the heat-set chamber which essentially eliminates temperature gradients throughout the length thereof and which has totally eliminated the dye streaking problem.